46 research outputs found
Enhancing Optical Gradient Forces with Metamaterials
We demonstrate how the optical gradient force between two waveguides can be
enhanced using transformation optics. A thin layer of double-negative or
single-negative metamaterial can shrink the interwaveguide distance perceived
by light, resulting in a more than tenfold enhancement of the optical force.
This process is remarkably robust to the dissipative loss normally observed in
metamaterials. Our results provide an alternative way to boost optical gradient
forces in nanophotonic actuation systems and may be combined with existing
resonator-based enhancement methods to produce optical forces with an
unprecedented amplitude.Comment: 5 pages, 4 figures; supplemental information available from AP
Linear CNNs Discover the Statistical Structure of the Dataset Using Only the Most Dominant Frequencies
We here present a stepping stone towards a deeper understanding of
convolutional neural networks (CNNs) in the form of a theory of learning in
linear CNNs. Through analyzing the gradient descent equations, we discover that
the evolution of the network during training is determined by the interplay
between the dataset structure and the convolutional network structure. We show
that linear CNNs discover the statistical structure of the dataset with
non-linear, ordered, stage-like transitions, and that the speed of discovery
changes depending on the relationship between the dataset and the convolutional
network structure. Moreover, we find that this interplay lies at the heart of
what we call the ``dominant frequency bias'', where linear CNNs arrive at these
discoveries using only the dominant frequencies of the different structural
parts present in the dataset. We furthermore provide experiments that show how
our theory relates to deep, non-linear CNNs used in practice. Our findings shed
new light on the inner working of CNNs, and can help explain their shortcut
learning and their tendency to rely on texture instead of shape
Three-Dimensional Measurement of the Helicity-Dependent Forces on a Mie Particle.
Recently, it was shown that a Mie particle in an evanescent field ought to experience optical forces that depend on the helicity of the totally internally reflected beam. As yet, a direct measurement of such helicity-dependent forces has been elusive, as the widely differing force magnitudes in the three spatial dimensions place stringent demands on a measurement's sensitivity and range. In this study, we report the simultaneous measurement of all components of this polarization-dependent optical force by using a 3D force spectroscopy technique with femtonewton sensitivity. The vector force fields are compared quantitatively with our theoretical calculations as the polarization state of the incident light is varied and show excellent agreement. By plotting the 3D motion of the Mie particle in response to the switched force field, we offer visual evidence of the effect of spin momentum on the Poynting vector of an evanescent optical field
LUCID-GAN: Conditional Generative Models to Locate Unfairness
Most group fairness notions detect unethical biases by computing statistical
parity metrics on a model's output. However, this approach suffers from several
shortcomings, such as philosophical disagreement, mutual incompatibility, and
lack of interpretability. These shortcomings have spurred the research on
complementary bias detection methods that offer additional transparency into
the sources of discrimination and are agnostic towards an a priori decision on
the definition of fairness and choice of protected features. A recent proposal
in this direction is LUCID (Locating Unfairness through Canonical Inverse
Design), where canonical sets are generated by performing gradient descent on
the input space, revealing a model's desired input given a preferred output.
This information about the model's mechanisms, i.e., which feature values are
essential to obtain specific outputs, allows exposing potential unethical
biases in its internal logic. Here, we present LUCID-GAN, which generates
canonical inputs via a conditional generative model instead of gradient-based
inverse design. LUCID-GAN has several benefits, including that it applies to
non-differentiable models, ensures that canonical sets consist of realistic
inputs, and allows to assess proxy and intersectional discrimination. We
empirically evaluate LUCID-GAN on the UCI Adult and COMPAS data sets and show
that it allows for detecting unethical biases in black-box models without
requiring access to the training data.Comment: 24 pages, 6 figures, 1st World Conference on eXplainable Artificial
Intelligenc
Tunable terahertz frequency comb generation using time-dependent graphene sheets
We investigate the interaction between electromagnetic pulses and two-dimensional current sheets whose conductivity is controlled as a function of time by the generation of photocarriers, and we discuss its applicability to tunable frequency comb generation. To this aim, we develop an analytical model that permits the calculation of the scattered waves off a thin sheet with time-dependent, dispersive sheet conductivity. We evaluate the transmitted spectrum as a function of the dispersive behavior and the modulation frequency of the number of photocarriers. We conclude that such active materials, e.g., time-dependent graphene sheets, open up the possibility to manipulate the frequency of incident pulses and, hence, could lead to highly tunable, miniaturized frequency comb generation
Do Optomechanical Metasurfaces Run Out of Time?
Artificially structured metasurfaces make use of specific configurations of subwavelength resonators to efficiently manipulate electromagnetic waves. Additionally, optomechanical metasurfaces have the desired property that their actual configuration may be tuned by adjusting the power of a pump beam, as resonators move to balance pump-induced electromagnetic forces with forces due to elastic filaments or substrates. Although the reconfiguration time of optomechanical metasurfaces crucially determines their performance, the transient dynamics of unit cells from one equilibrium state to another is not understood. Here, we make use of tools from nonlinear dynamics to analyze the transient dynamics of generic optomechanical metasurfaces based on a damped-resonator model with one configuration parameter. We show that the reconfiguration time of optomechanical metasurfaces is not only limited by the elastic properties of the unit cell but also by the nonlinear dependence of equilibrium states on the pump power. For example, when switching is enabled by hysteresis phenomena, the reconfiguration time is seen to increase by over an order of magnitude. To illustrate these results, we analyze the nonlinear dynamics of a bilayer cross-wire metasurface whose optical activity is tuned by an electromagnetic torque. Moreover, we provide a lower bound for the configuration time of generic optomechanical metasurfaces. This lower bound shows that optomechanical metasurfaces cannot be faster than state-of-the-art switches at reasonable powers, even at optical frequencies
Frequency converter implementing an optical analogue of the cosmological redshift
According to general relativity, the frequency of electromagnetic radiation
is altered by the expansion of the universe. This effect--commonly referred to
as the cosmological redshift--is of utmost importance for observations in
cosmology. Here we show that this redshift can be reproduced on a much smaller
scale using an optical analogue inside a dielectric metamaterial with
time-dependent material parameters. To this aim, we apply the framework of
transformation optics to the Robertson-Walker metric. We demonstrate
theoretically how perfect redshifting or blueshifting of an electromagnetic
wave can be achieved without the creation of sidebands with a device of finite
length.Comment: 6 pages, 3 figure